Optimum AFR for DI?

D

dadasracecar

Member
So I'm very new at tuning, modding, and this whole forum thing. This is the first forum I've ever subscribed to, the speed6 is my first turbo car, car that I've tuned and modded. Crazy, I know, considering what I've done with this car. Anyway, I'm perplexed about the optimum afr for our cars so I've been poking around and found a couple of things but I know that other with more experience on forums will be able to find more information regarding the proper afr for DI engines. Please help. I think it would also be VERY interesting for someone with a stock speed6 and a dashhawk to log the afr in a WOT run in fourth. Maybe we're supposed to be in the 12-13 range at low rpms...

Here's a crazy high afr from a stock 335i: http://www.e90post.com/forums/showthread.php?t=29708&highlight=afr

Yikes: http://www.e90post.com/forums/showthread.php?t=105342

once again: http://www.e90post.com/forums/showthread.php?t=105080

on 91 octane huh.gif http://www.e90post.com/forums/showthread.php?t=104489
 
Last edited:
Dada,

I plan on getting a dashhawk this week. Right now, I am back to stock except for the motor mount, so hopefully, within 2 weeks I will have some datalogs for you.
 
dadasracecar said:
So I'm very new at tuning, modding, and this whole forum thing. This is the first forum I've ever subscribed to, the speed6 is my first turbo car, car that I've tuned and modded. Crazy, I know, considering what I've done with this car. Anyway, I'm perplexed about the optimum afr for our cars so I've been poking around and found a couple of things but I know that other with more experience on forums will be able to find more information regarding the proper afr for DI engines. Please help. I think it would also be VERY interesting for someone with a stock speed6 and a dashhawk to log the afr in a WOT run in fourth. Maybe we're supposed to be in the 12-13 range at low rpms...

Here's a crazy high afr from a stock 335i: http://www.e90post.com/forums/showthread.php?t=29708&highlight=afr

Yikes: http://www.e90post.com/forums/showthread.php?t=105342

once again: http://www.e90post.com/forums/showthread.php?t=105080

on 91 octane huh.gif http://www.e90post.com/forums/showthread.php?t=104489
Click to expand...


Hey Jason, I'm looking into the secondary air injection pump Shiv mentioned. Apparently some cars still do use them to this day. I'll let you know if I find anything juicy...
 
www.cp-e.com said:
Hey Jason, I'm looking into the secondary air injection pump Shiv mentioned. Apparently some cars still do use them to this day. I'll let you know if I find anything juicy...
Click to expand...

the n/a 2.0l 8v from vw used a secondary air system with an EGR valve off of the exhaust side of the head, but afr weren't so lean as they are with that bmw post.

edit: those all seem high, but not too crazy high. the highest i would tune for would be 12.8:1.
 
mrlilguy157 said:
the n/a 2.0l 8v from vw used a secondary air system with an EGR valve off of the exhaust side of the head, but afr weren't so lean as they are with that bmw post.

edit: those all seem high, but not too crazy high. the highest i would tune for would be 12.8:1.
Click to expand...


That's funny, VW was exactly the example I was thinking of. One of our employees is big into VW and he mentioned that some of them still rely on air injection. And here I thought air injection was more or less "replaced" by the newer cats that store and release oxygen.

Taken from Wikipedia:

Wikipedia said:
Many three-way catalytic converters utilize an air injection tube between the first (NOx reduction) and second (HC and CO oxidation) biscuits of the converter. This tube is fed by either an air pump or by an aspirator. The injected air provides oxygen for the catalyst's oxidizing reaction. These systems also sometimes include an upstream air injector to admit oxygen to the exhaust system before it reaches the catalytic converter. This precleans the extra-rich exhaust from a cold engine, and helps bring the catalytic converter quickly up to operating temperature.

Most newer systems do not employ air injection. Instead, they provide a constantly varying mixture that quickly and continually cycles between lean and rich to keep the first catalyst (NOx reduction) from becoming oxygen loaded, and to keep the second catalyst (CO oxidization) sufficiently oxygen-saturated. They also utilize several oxygen sensors to monitor the exhaust, at least one before the catalytic converter for each bank of cylinders, and one after the converter. Some systems contain the reduction and oxidation functions separately rather than in a common housing.
Click to expand...


I'm trying to understand if there is in fact an air injection system in the 335i, or if they are actually running these cars that lean. I'm leaning towards air injection, but I'm intrigued by their numbers nonetheless.
 
Well if you go back to the olden days, lol, the 5.0 Mustang. They too had air injection into the catalyst. The air tube came in AFTER the pre-cats. If you omitted the cats with an offroad pipe, your air injection tube would flop in the wind. If the Wideband is reading the AFR before the air injection, than they are indeed running crazy lean. If its AFTER the air injection, than that could account for the higher readings. Only one way to confirm, slap an aftermarket wide bad before the primary cats.
 
RX-8s still have air injection, but only on warm-up. I imagine there may be emissions countermeasures out there that use supplemental air.

A/F is A/F - the gas doesn't care how it got into the motor.

Gasoline makes power from 11.5:1 up to 13.3:1.
Where you put your A/F depends on how heavily you are relying on it for things other than power.
If any motor made a ton of power at 14:1 (and survived), it will make two tons of power at 12:1.
 
Power drops as the AFR is lower my friend. You are reducing the amount of air at a lower AFR ratio, which causes the fuel to burn slower. Slower burn = less heat. Higher AFR = More heat, which equates to lean. YOu do need to watch AFR for tuning, but you also need to consider boost pressure, cfm, etc as well... not to mention, timing!
 
Actually, the flame front speeds up as you get richer until the RBT point where it slows down again. 12:1 burns much faster than 13:1.
Whether power goes up or down depends on when maximum pressure is created.
 
mk5 GTi dyno's with upgraded fuel pumps:

TheBox said:
APR stage 2+ FP performance file/ FP/ IC / TBE and VFE intake

271 HP abd 322 TQ !!!!!!

jrdynograph005.jpg
Click to expand...

VF Turbo Kit with pump

t3t41.8tgti said:
I went ahead and replotted the dyno sheets with rpm instead of mph. The first run was done on the pump gas program with 93 octane and the second run was done one the race gas program with 103 octane. As you can see the boost doesn't change between the 2 programs. The car has the following mods if you are interested.
VF RSS turbo kit with gt2860rs turbo
AWE turboback exhaust w/ metal cat
VF FMIC
Spec stage 3 clutch
Spec aluminum flywheel.

RSSracevspumprpm.jpg
Click to expand...

TheBox said:
Well here it is the dyno some of you have asked for.... APR stage 2+ , FP, IC and TBE 93 oct.

I put c16 in the tank to do a race file pull and something happended while switching to 100 oct.. but i am def happy coreccted sae was 253 HP.

Sorry no torque the dyno was too old and didn't have a digital pickup

oh and that little blit in the begining was the tires sppinging on the dyno!!! <IMG NAME="icon" SRC="http://www.vwvortex.com/zeroforum_graphics/biggrinsanta.gif" BORDER="0">

dyno.jpg
Click to expand...

rysskii3 said:
first it was installed by the good guys at group5motorsport.com

the installation looked good and they did a very good job in about an hour. (this was the 1st time they did something like that). I drove normal for about 10 min. everything seemed fine. Then I decided to push it and see how it would go. So in 2nd gear at about 3500 I punched it hard. Nothing but wheel spin through the whole gear. I hit 3rd, wheel spin again. So I let out a lil and punched it again at 4000 in 3rd. GOD DAMN! This thing pulled like a monster. I dont even have words to describe it. So I hit 4th and all the sudden Im at 120. Holly sh*t the car was so strong. Just unbelievable. I didnt think that it would give me that much power, but it did. Very impressed with the kit. For the price it is, a well worth mod. I dont even know how to explain it. My heart was just pounding the while time while I was on the throttle. :eek:

Thank you AUTOTECH and GROUP5MOTORSPORT.com for all the fun I had tonight.
Click to expand...
 
Please log in or register to reply here.

Similar threads

D
Optimum AFR for DI?
Replies
18
Views
4K
matsuda
M
Y
Stumbling/Sputtering Under Acceleration
Replies
10
Views
4K
Jaleik
J
L
MP5 no start condition.
Replies
2
Views
1K
lilnate1191
L

Latest posts

Back